Hybrid imaging units are increasingly gaining importance in the field of medical imaging, this being so because they make it possible to be able to examine a patient in a very short time, sometimes even without repositioning, with the aid of two different modalities, that is to say to be able to compile image information with the aid of two different imaging devices. Such hybrid imaging units in this case include a first imaging device of high spatial resolution, for example a computer tomograph or a magnetic resonance machine, and a second, in the present case, nuclear medicine imaging device of high sensitivity for example for PET (position emission tomography) or SPECT (single photon emission computed tomography).
Both methods are tomographic methods that show in the body the distribution of a radionuclide, that is to say a radiopharmaceutical, given to the patient. Such radionuclides have the property of accumulating intensively at specific pathological zones. PET or SPECT imaging methods permit the acquisition of the radionuclide distribution in the body, while corresponding images that finally show probability distributions and constitute an “activity card” can be determined from the acquired measurement signals and displayed. The mode of operation of these methods is known in principle, and there is no need to go into this in more detail.
It is particularly expedient to combine a first imaging device in the form of a magnetic resonance machine with a second imaging device in the form of a PET device. The point is that magnetic resonance tomography permits a very high spatial resolution, on the one hand, while not influencing the PET measurement, on the other hand. As a result, it is possible to arrange the PET detectors in the interior of the cylindrical patient aperture of a conventional magnetic resonance system such that both can measure using the same isocenter, and is even possible for both measurements to run simultaneously. The PET examination furthermore delivers very informative images, and this is to be ascribed to the production of the measurement signals (time-resolved detection of gamma quanta). What is involved here is a coincidence measurement method of high counting yield and thus of very high sensitivity.
The combination of a magnetic resonance system with a PET or SPECT imaging device is particularly advantageous, since there is the possibility here of simultaneously measuring with both modalities, since the two modalities are based on completely different measurement principles and do not influence one another. The PET or SPECT detectors are arranged in the interior of the patient aperture of the magnetic resonance system, and thus clad the latter such that the PET or SPECT imaging device and the magnetic resonance imaging device can measure isocentrically and scan the same examination volume.
However, it disadvantageous in this case that the protocols of the various imaging modalities that are processed in the course of the respective measurement and provide the basis for the respective measurement by determining the parameters have different measuring times, that is to say, for example, the PET measurement proceeds more quickly than the magnetic resonance measurement. The PET image is therefore acquiring no further data while the MR measurement is still running. Conversely, the magnetic resonance system is idle when it has finished before the end of the PET measurement. The respective imaging device is therefore fundamentally ready to operate but is not working because the original measurement protocol has already been completely processed. This is, however, inefficient with regard to the level of capacity utilization of the hybrid imaging unit.